The present invention relates to compound gear pumps and engine hydraulic circuits using same.
One of the compound gear pumps is disclosed in JP-U 50-114705. This pump is of the external gear type including a crescent-type internal gear pump so called having a crescent between inner and outer rotors wherein the difference in number of teeth therebetween is two or more, and a third rotor having external teeth circumscribed with external teeth of the outer rotor.
Specifically, the outer rotor having 31 internal teeth eccentrically disposed to and inscribed with the inner rotor having 24 external teeth is rotatably arranged in a circular large-diameter concavity formed in a casing. The outer rotor has also 31 external teeth arranged axially on the whole outer periphery, which are meshed with the 12 external teeth of the third rotor rotatably arranged in a circular small-diameter concavity continuously formed with the large-diameter concavity. The outer rotor is rotatably held in the large-diameter concavity through slide contact of the top of its external teeth with a wall of the large-diameter concavity.
With the known compound gear pump, however, since rotatable holding of the outer rotor is ensured by slide contact of the top of its external teeth with the wall of the large-diameter concavity, the pressure on a contact face of the top of the external teeth may be increased to produce wear of the inner periphery of the casing and the top of the external gear, resulting in, at worst, seizing of the two.
Moreover, since the difference in number of teeth is great between the inner rotor having 24 external teeth and the outer rotor having 31 internal teeth, the outer rotor is quite lower in number of revolutions than the inner rotor, having substantially 2/3 the number of revolutions of the inner rotor. This causes lowered number of revolutions of the third rotor driven by the outer rotor. Thus, in order to secure a predetermined discharge of an external gear pump, the external teeth of the outer rotor meshed with those of the third rotor should be increased in width or height. Such increase in width or height of the external teeth is accompanied with a reduction in thickness of the outer rotor. In view of the fact that the compound gear pump has the same numbers of the inner and outer teeth on the inner and outer peripheries of the outer rotor, the strength of the outer rotor should be secured by avoiding the inner and outer teeth radially overlapping each other as described in the above reference. As a consequence, the degree of freedom is decreased with regard to a design of the external teeth of the outer rotor, causing a problem of difficult determination of the optimum specification of the external gear pump.
It is, therefore, an object of the present invention to provide compound gear pumps which are free from wear and seizing, and allow easy determination of their optimum specifications.
Another object of the present invention is to provide engine hydraulic circuits using the compound gear pumps.